Photodissociation dynamics of N,N-dimethylformamide at 225 nm and 245 nm

Abstract

N,N-Dimethylformamide, (CH₃)₂NCHO, is the simplest tertiary amide and a model compound for investigating the photofragmentation of peptide bonds. We report the results of a velocity-map imaging study into the photodissociation dynamics of DMF following excitation at 225 nm and 245 nm. Excitation at either wavelength generates a variety of products, with the primary dissociation pathways involving cleavage of either the N–CO amide bond or an N–CH₃ bond. Excitation at 225 nm is predominantly to the S₂ 2¹A′′ state via a parallel transition, with dissociation of the amide bond occurring either on this state or on a lower singlet surface following internal conversion. The topographies of all of the potential energy surfaces involved result in dissociation from a range of planar (apart from the methyl-group hydrogen atoms) and non-planar molecular geometries. Dissociation from planar geometries leads to little product internal excitation, correspondingly high photofragment velocities, and near-limiting values of the recoil-anisotropy parameter β. Dissociation from non-planar geometries leads to significant product internal excitation, with correspondingly lower photofragment velocities and breakdown of the axial recoil approximation to give reduced values of β. Excitation at 245 nm involves the same excited-state surfaces, but at the longer wavelength the S₂ state can only be reached from non-equilibrium geometries of the ground state, leading to a reduction in the recoil anisotropy parameter relative to excitation at 225 nm. The potential energy curves associated with cleavage of the N–CH₃ bond are less well characterised. However, the pathway is characterised by an isotropic angular distribution and a TKER distribution peaking at low energies, both of which can be rationalised in terms of the molecular geometry and the orientation of the transition dipole involved in the excitation step.